Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii, is considered the most severe of the human rickettsioses, attacking both endothelial and smooth muscle cells of small blood vessels. The purpose of this investigation is to characterize this infection in cultured human endothelial cells and to establish possible correlations between in vitro observations of the infection and the pathogenesis of the disease in man. Specifically, cultured vascular endothelial cells derived from human umbilical veins will be infected with R. rickettsii. The infection will be characterized by light microscopy to determine basic growth characteristics and spreading capabilities of the organism. Transmission electron microscopy will be used to: (a) establish mode of entry of the organism (active penetration vs. phagocytosis); (b) determine the relationship of the organism to cellular organelles; (c) assess host cell cytopathology; and (d) study the effects of crude rickettsial extracts and purified R. rickettsii endotoxin on cellular fine structure. Scanning electron microscopy will be used to determine whether R. rickettsii-infected endothelial cells bind human platelets to their surfaces. The capacity of centrifuged supernatant culture fluids removed from infected endothelial cells to aggregate added platelets will be assessed and, subsequently, the levels of prostaglandin I2 (prostacyclin), an extremely potent and stable inhibitor of platelet aggregation will be measured in infected culture fluids. Two lysosomal enzymes, acid phosphatase and beta-glucuronidase, whose activity is known to be stimulated by the lipopolysaccharide moiety of bacterial endotoxin will be monitored both in culture supernatant fluids and in crude cell extracts from infected and uninfected cells.